1. Field of the Invention
This invention relates to fast framing thermal imaging systems commonly referred to as Forward Locking Infra-Red systems. More particularly, this invention relates to the scanner mechanism of the thermal imaging system.
2. Description of the Prior Art
Presently there exist many thermal imaging systems designed to convert infrared radiation to visible radiation for viewing by an observer. The most common types of these thermal imaging systems are single framing thermographic cameras, downward looking single channel thermal mapping systems, and fast framing thermal imaging systems.
Fast framing thermal imaging systems comprise mechanically-scanning devices which convert radiation in the far infrared spectral region to visible radiation in real time and at an information rate (or frame rate) comparable to that of standard television. Such systems are commonly referred to as FLIR systems, the acronym for Forward Looking Infra-Red. Although the term FLIR originally implied an airborne system, it is now used to denote any fast framing thermal imager. Thermal imaging in a FLIR is produced by an optical system which collects, spectrally filters, and focuses the infrared scene radiation onto an optically scanned multi-element detector array. The elements of the detector array then convert the optical signals into analog electrical signals which are amplified and processed for display on a monitor such as a video monitor.
The function of the scanner mechanism in a FLIR is to move the image formed by the optical system in the plane of the detector array in such a way that the detectors dissect the image sequentially and completely. There are two basic types of scanners; a parallel beam scanner which consists of an optical angle changing device such as a moving mirror placed in front of the final image forming lens, and a converging beam scanner which consists of a moving mirror or other scanning device placed between the final lens and the image. The seven most commonly used optical scanning mechanisms include the oscillating mirror, the rotating polygonal mirror, the rotating refractive prism, the rotating wedge, the revolving lens, the rotating sensor, and the rotating V-mirror. One-dimensional or two-dimensional scanners may be implemented by various combinations of the above scanning mechanisms.
In addition to the distinctions between scanning mechanism types such as convergent and parallel beam scanning, and one- or two-dimensional scanning, there is a distinction between serial and parallel scene dissection and detector signal processing. In parallel scene dissection, an array of detectors is oriented perpendicular to the primary scan axis, as in a unidimensional detector array used with an azimuth scanner. All of the detector outputs are amplified, processed and displayed simultaneously or in parallel. In serial scene dissection, an array of detectors is oriented parallel to the primary scan access and each point of the image is scanned by all of the detectors. The detector outputs are then appropriately delayed and summed by an integrating delay line which superimposes the outputs, thereby simulating a single scanning detector, or they may be read out, one-for-one, on a similar array of scanning display elements such as LED elements. From the foregoing, it should be appreciated that there exist many different types of FLIRs, with the particular type of scanning selected being dependent upon such factors as overall allowed sensor size, allowed power consumption, and performance-to-cost ratio.
As an example, one of the simplest state-of-the-art FLIRs that attempts to optimize the above factors is a parallel scan parallel video system which requires only a collecting converging optic, a two-sided oscillating scanner, a detector array, amplifying electronics, display drivers, and an eyepiece. More particularly, in one specific embodiment of such a FLIR, the infrared energy from the viewed scene or target is received by an afocal, magnifying, infrared lens having a 3:1 step zoom capability. The recollimated beam from the afocal lens impinges upon the front surface of the scanner which reflects the infrared energy to the detector array. The outputs of the detector array are amplified and shaped to appropriately drive the visible light emitters, such as an LED array having a one-to-one correspondence to the individual elements of the detector array. The visible output from the LED array is then reflected off the back side of the scanner to a visible optic which magnifies and focuses the reflected visible scan for viewing by the observer.
The primary advantages to the above mentioned of a simple FLIR so that no scan synchronizer is needed to synchronize the thermal scan with the visible scan, the scanner requires a relatively low amount of power, and the display is compact. The minor disadvantages of the FLIR are that only one observer may observe the display, loss of any part of a channel causes loss of one line of video, all channels must be balanced individually, all channels must be controlled simultaneously (ganged), the channels may require d.c. restoration, and video waveform shaping must be performed in each channel. Fortunately, well-known electronic techniques have been developed to compensate for the above disadvantages.
The major disadvantages to the above described FLIR include the need for an oscillating scanner, the requirement for a large number of detector elements constituting the detector array, and the lack of reliable synchronization between the oscillating scanner and the sampling of the detector elements. More particularly, such oscillating scanners typically comprise a flat mirror which oscillates in the azimuthal direction enabling the detector array to sequentially dissect the thermal image. It should be appreciated that the mirror must scan in the azimuthal direction, stop, and then scan in the reverse azimuthal direction. Obviously, a high torque motor is required to almost instantaneously decelerate the angular velocity of the mirror at the end of the first frame, and then to almost instantaneously accelerate the mirror to scan the second frame to achieve a relatively constant rate of scan (dwell time). In actual practice, the dwell time cannot be held constant due to the fact that it is virtually impossible to instantaneously decelerate and accelerate the mirror. This results in a substantial amount of mechanical jitter in the scan which causes a substantial information delay at the output of the detectors. Moreover, the reverse scan in the opposite azimuthal direction typically accentuates such a delay by a factor of two thereby causing considerable blurring of the resultant picture. Appropriate phase shift techniques must therefore be provided to compensate for such a delay factor. It is noted that the amount of mechanical jitter associated with oscillatory mirror mechanisms is substantially increased when the FLIR is used in actual working conditions and environments. As a result, FLIRs are less rugged than would be normally desired. Additionally, the oscillatory mirror mechanism creates a great amount of microphonics which adversely affects the performance of the FLIR.
As noted earlier, another major disadvantage of the FLIR system described above is the requirement for a large number of detector elements which constitute the detector array. In order to completely dissect the thermal image, the individual detector elements must be vertically stacked immediately adjacent to one another. Present state-of-the-art manufacturing techniques have been unable to fabricate the detector elements adjacent to one another a distance no closer than 25 microns. This results in incomplete dissection of the thermal image. In order to compensate for the space between the detector elements, it has been found necessary to stagger a second vertical column of detector elements adjacent to the first row of detector elements. Obviously, such staggered arrays double the amount of detector elements (and also doubles the amount of associated electronics) needed to completely dissect the thermal image. It is well-known that the cost of the FLIR system increases at least parabolically in proportion to the amount of detector elements needed to completely dissect a thermal image. Moreover, the cost of a fully operable detector array increases dramatically when the individual detector elements are fabricated close to one another. This is due to the inability to repair by "cut and paste" techniques, inoperable detector elements of the detector array. Thus, the entire detector array must be scrapped in the event a single detector element is improperly fabricated or in the event a single detector element becomes inoperable during use.
Recently, it has been found that the amount of detector elements described above which are needed to completely dissect the thermal image can be reduced by one-quarter by using interlacing techniques. Specifically, in these improved FLIRs, a 2:1 interlace of the thermal image is accomplished by providing a means for tilting the oscillating mirror in a vertical (elevational) direction and by fabricating the detector array in a single vertical column with each detector element being spaced apart from its adjoining elements by a distance equal to its pel size. In operation, the 2:1 interlace scan of the thermal image is accomplished by scanning the mirror in an azimuthal direction to complete one field of the frame, pivoting the mirror in a vertical direction by a distance equal to the pel size of the detector elements, scanning the mirror in the reverse azimuthal direction to complete the second field of the frame, and then pivoting the mirror in the reverse vertical direction to complete one frame. It should be appreciated that such 2:1 interlacing techniques eliminate the need for a two column staggered detector array, thereby decreasing the number of detector elements needed to completely dissect the image by one-quarter. Unfortunately, the need for pivoting the mirror in a vertical direction in addition to the azimuthal direction doubles the inherent disadvantages to the oscillating mirror and results in a substantial amount of mechanical jitter in the vertical direction in addition to the azimuthal direction.
Finally, the third major disadvantage to the simplified FLIR and to the improved FLIR discussed above, is the difficulty of synchronizing the oscillating mirror with the rate in which the detector elements are sampled as the detector array dissects the thermal image. The only available method for synchronizing the oscillating mirror with the sampling rate is to incorporate a position transducer within the gimbal of the oscillating mirror which senses the completion of the scanning of each field. This information is then compared with an electronically generated position command, the difference of which is amplified to send corrective signals to the torque motors controlling the oscillation of the mirror. Obviously, inasmuch as the transducer is only able to sense the completion of the scanning of each field, there exists no method for synchronizing the sampling rate with any variation in the scan rate. Thus, as noted earlier, the detector elements cannot be linearly sampled in relation to the scanning of the thermal image.
Therefore it is an object of this invention to provide an apparatus and method which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the art of thermal imaging systems.
Another object of this invention is to provide an apparatus and method for scanning linear infrared detector arrays which eliminates mechanical jitter commonly associated with oscillatory scanning devices.
Another object of this invention is to provide an apparatus and method for scanning linear infrared detector arrays in which the thermal image is scanned onto the detector array in a smooth and continuous motion resulting in reduced microphonics and in a significantly more rugged FLIR, the performance of which is unaffected when he FLIR is used in less than favorable conditions and environments.
Another object of this invention is to provide an apparatus and method for scanning linear infrared detector arrays in such a manner that the frame of the image is sequentially and completely dissected by the detector array with a minimal amount of detector elements constituting the detector array to substantially reduce the cost of the FLIR while providing a greatly strengthened system.
Another object of this invention is to provide an apparatus and method for scanning linear infrared detector arrays by using 2:1 interlacing techniques which enable the individual detector elements to be spaced apart from one another a distance sufficient to facilitate repair of inoperable detector elements by "cut and paste" techniques thereby eliminating the need for scrapping the entire detector array when only one detector element is inoperable.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.